This invention relates to non-carbon, metal-based, anodes for use in cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing molten electrolyte such as cryolite, and to methods for their fabrication, as well as to electrowinning cells containing such anodes and their use to produce aluminium.
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950xc2x0 C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Hxc3xa9roult, has not evolved as many other electrochemical processes.
The anodes are still made of carbonaceous material and must be replaced every few weeks. During electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting CO2 and small amounts of CO and fluorine-containing dangerous gases. The actual consumption of the anode is as much as 450 Kg/Ton of aluminium produced which is more than ⅓ higher than the theoretical amount of 333 Kg/Ton.
Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production.
U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained by the addition of cerium to the molten cryolite electrolyte. This made it possible to have a protection of the surface only from the electrolyte attack and to a certain extent from the gaseous oxygen but not from the nascent monoatomic oxygen.
EP Patent application 0 306 100 (Nyguen/Lazouni/Doan) describes anodes composed of a chromium, nickel, cobalt and/or iron based substrate covered with an oxygen barrier layer and a ceramic coating of nickel, copper and/or manganese oxide which may be further covered with an in-situ formed protective cerium oxyfluoride layer.
Likewise, U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium production anodes with an oxidised copper-nickel surface on an alloy substrate with a protective oxygen barrier layer. However, full protection of the alloy substrate was difficult to achieve.
Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. As mentioned hereabove, many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry.
A major object of the invention is to provide an anode for aluminium electrowinning which has no carbon so as to eliminate carbon-generated pollution and increase the anode life.
A further object of the invention is to provide an aluminium electrowinning anode material with a surface having a high electrochemical activity for the oxidation of oxygen ions for the formation of bimolecular gaseous oxygen and a low solubility in the electrolyte.
Another object of the invention is to provide an anode for the electrowinning of aluminium which is covered with an electrochemically active layer with limited ionic conductivity for oxygen ions.
Yet another object of the invention is to provide an anode for the electrowinning of aluminium which is made of readily available material(s).
An important object of the invention is to substantially reduce the solubility of the surface layer of an aluminium electrowinning anode, thereby maintaining the anode dimensionally stable.
Yet another object of the invention is to provide operating conditions for an aluminium electrowinning cell under which the contamination of the product aluminium is limited.
The invention is based on the fact that iron-nickel alloys when oxidised form a dense and coherent oxide layer consisting essentially of iron oxide, in particular hematite. As this oxide layer is well adherent to the non-oxidised iron-nickel alloy and also electrochemically active for the oxidation of oxygen ions, it can be used as an electrochemically active surface for the oxidation of oxygen ions of an anode for the electrowinning of aluminium. Small scale tests have also shown that such an iron oxide-based layer has a slow dissolution rate in fluoride-containing molten electrolyte which can even be substantially suppressed under favourable cell operating conditions.
Therefore, the invention relates to an anode of a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing molten electrolyte. The anode comprises an iron-nickel alloy body or layer whose surface is oxidised to form a coherent and adherent outer iron oxide-based layer, in particular a hematite-based layer, the surface of which is electrochemically active for the oxidation of oxygen ions and which reduces diffusion of oxygen from the electrochemically active surface into the iron-nickel alloy body or layer.
The surface oxidation of the iron-nickel alloy body may be such as to form an iron oxide-based layer comprising a dense iron oxide outer portion, a microporous iron oxide portion which separates the outer portion from a two-phase inner portion, one phase containing iron oxide, the other phase containing a nickel metal.
The surface of the iron-nickel alloy body or layer may be oxidised in a molten electrolyte at 800 to 1000xc2x0 C. for 5 to 15 hours. Alternatively, the surface of the iron-nickel alloy body or layer may be oxidised at 750 to 1150xc2x0 C. for 5 to 100 hours, in particular 20 to 75 hours at average temperature or below 25 hours at elevated temperature, in an oxidising atmosphere such as air or oxygen.
Usually, the iron-nickel alloy body or layer comprises 50 to 95 weight % iron and 5 to 50 weight % nickel, preferably 50 to 80 weight % iron and 20 to 50 weight % nickel, and even more preferably 60 to 70 weight % iron and 30 to 40 weight % nickel, i.e. with optionally up to 45 weight % of further constituents providing it is still capable of forming an iron oxide-based electrochemically active layer. Normally, the iron-nickel alloy comprises less than 30 weight %, in particular less than 20 weight % and often less than 10 weight %, of further constituents. Such constituents may be added to improve the mechanical and/or electrical properties of the anode substrate, and/or the adherence, the electrical conductivity and/or the electrochemical activity of the anode layer.
Alternatively, the iron-nickel alloy body or layer may comprise more than 50 weight % nickel, as described below.
The iron-nickel alloy body or layer may in particular comprise in addition to iron and nickel the following constituents in the given proportions: up to 15 weight % of chromium and/or additional alloying metals selected from titanium, copper, molybdenum, aluminium, hafnium, manganese, niobium, silicon, tantalum, tungsten, vanadium, yttrium and zirconium, in a total amount of up to 5 weight %. Furthermore, nickel present in the iron-nickel alloy may be partly substituted with cobalt. The iron-nickel alloy may contain up to 30 weight % of cobalt.
The anode may comprise a layer of iron-nickel alloy on an oxidation resistant and preferably highly electrically conductive metallic core, such as copper or a copper alloy, possibly containing minor amounts of at least one oxide reinforcing the mechanical properties of the metallic core. The reinforcing oxides may be selected from alumina, hafnia, yttria and zirconia.
This metallic core may be coated with at least one metal selected from nickel, chromium, cobalt, iron, aluminium, hafnium, manganese, molybdenum, niobium, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium, and alloys, intermetallic compounds and combinations thereof.
The metallic core may be coated with an intermediate protective layer against oxidation.
A layer of iron-nickel alloy may be applied on an oxidation resistant metallic core before or after formation of said outer iron oxide-based layer. The iron-nickel alloy layer may be plasma sprayed, arc sprayed, chemically or electrochemically deposited on the metallic core.
Optionally, the iron-nickel alloy layer may be bonded to the metallic core through at least one intermediate layer, such as a film of silver and/or at least one layer of nickel and/or copper.
The invention also relates to a bipolar electrode of a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing electrolyte, comprising on its anodic side an anode as described above.
Another aspect of the invention is a method of manufacturing an anode as described above. The method comprises: providing an iron-nickel alloy body or layer; and oxidising the surface of the iron-nickel alloy body or layer to form a coherent and adherent outer iron oxide-based layer the surface of which is electrochemically active for the oxidation of oxygen ions.
When a nickel-rich iron-nickel- alloy body or layer, i.e. having a nickel content above 50 weight %, in particular between 60 and 80 weight %, is pre-oxidised to manufacture an anode, a composite oxide layer may form on the alloy body or layer. Such a composite oxide layer usually comprises an iron oxide-rich electrochemically active outer layer separated by a nickel ferrite-rich intermediate layer from the iron-nickel alloy body or layer. The nickel-ferrite intermediate layer acts as a selective membrane in the sense that it inhibits subsequent oxygen diffusion to the alloy body or layer but permits migration of iron metal from the alloy body or layer towards the electrochemically active outer layer thereby inhibiting direct oxidation of the alloy body or layer during use.
Therefore, the invention relates also to an anode of an aluminium electrowinning cell which comprises a nickel-iron alloy-containing body or layer, an electrochemically-active iron oxide-based outside layer, in particular a hematite layer, and a nickel-ferrite selective membrane between the iron oxide-containing outside layer and the nickel-iron alloy-containing body or layer. The nickel-ferrite selective membrane prevents oxidation of the nickel-iron alloy-containing body or layer but permits migration of iron metal from the nickel-iron alloy-containing body or layer to the iron oxide-containing outside layer where the migrated iron metal is oxidised to form iron oxide. The nickel-ferrite selective membrane is formed by surface oxidation of the nickel-iron alloy-containing body or layer.
The nickel-iron alloy-containing body or layer may comprise a nickel-iron weight ratio greater than 1, in particular from 1.5 to 4.
A further aspect of the invention is a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing electrolyte comprising at least one anode as described above.
During normal operation the electrochemically active layer of the or each anode may be progressively further formed by surface oxidation of the iron-nickel alloy body or layer by controlled oxygen diffusion through the electrochemically active layer, and progressively dissolved into the electrolyte at the electrolyte/anode interface, the rate of formation of the outer iron oxide-based layer being substantially equal to its rate of dissolution into the electrolyte.
Alternatively, it has been observed that this type of anode may be maintained dimensionally stable under specific cell operating conditions.
In known processes, even the least soluble anode material releases excessive amounts of constituents into the bath, which leads to an excessive contamination of the product aluminium. For example, the concentration of nickel (a frequent component of proposed metal-based anodes) found in aluminium produced in small scale tests at conventional cell operating temperatures is typically comprised between 800 and 2000 ppm, i.e. 4 to 10 times the maximum acceptable level which is 200 ppm.
Iron oxides and in particular hematite (Fe2O3) have a higher solubility than nickel in molten electrolyte. However, in industrial production the contamination tolerance of the product aluminium by iron is also much higher (up to 2000 ppm) than for other metal impurities.
Solubility is an intrinsic property of anode materials and cannot be changed otherwise than by modifying the electrolyte composition and/or the operating temperature of a cell.
Small scale tests utilising a NiFe2O4/Cu cermet anode and operating under steady conditions were carried out to establish the concentration of iron in molten electrolyte and in the product aluminium under different operating conditions.
In the case of iron oxide, it has been found that lowering the temperature of the electrolyte decreases considerably the solubility of iron species. This effect can surprisingly be exploited to produce a major impact on cell operation by limiting the contamination of the product aluminium by iron.
Thus, it has been found that when the operating temperature of the cell is reduced below the temperature of conventional cells (950-970xc2x0 C.) an anode covered with an outer layer of iron oxide can be made dimensionally stable by maintaining a concentration of iron species and alumina in the molten electrolyte sufficient to reduce or suppress the dissolution of the iron-oxide layer, the concentration of iron species being low enough not to exceed the commercial acceptable level of iron in the product aluminium.
The presence of dissolved alumina in the electrolyte at the anode surface has a limiting effect on the dissolution of iron from the anode into the electrolyte, which reduces the concentration of iron species necessary to substantially stop dissolution of iron from the anode.
Therefore, anodes according to the invention may be kept dimensionally stable by maintaining a sufficient amount of dissolved alumina and iron species in the electrolyte to reduce or prevent dissolution of the outer oxide layer.
The cell should be operated at a sufficiently low temperature to limit the solubility of iron species in the electrolyte, thereby limiting the contamination of the product aluminium by constituents of the outer iron oxide-based layer of the anode(s) to a commercially acceptable level.
When the cell is operated with a fluoride-based melt the operating temperature of the electrolyte should be above 700xc2x0 C., usually from 820 to 870xc2x0 C.
The amount of iron species and alumina dissolved in the electrolyte preventing dissolution of the iron oxide-based outside surface layer of the or each anode should be such that the product aluminium is contaminated by no more than 2000 ppm iron, preferably by no more than 1000 ppm iron, and even more preferably by no more than 500 ppm iron.
Usually the iron species are intermittently fed into the electrolyte, for instance together with alumina, to maintain the amount of iron species in the electrolyte constant which, at the operating temperature, prevents the dissolution of the iron oxide-based outside surface layer of the anodes.
However, the iron species can also be a continuously fed, for instance by dissolving a sacrificial electrode which continuously feeds the iron species into the electrolyte.
An electrical voltage may be applied to the sacrificial electrode. The applied voltage should be lower than the voltage of oxidation of oxygen Oxe2x80x94. An electrical current may be supplied to the sacrificial electrode to control and/or promote the dissolution of the sacrificial electrode into the electrolyte. The electrical current may be adjusted so that it corresponds to a current necessary for the dissolution of the required amount of iron species into the electrolyte replacing the iron which is cathodically reduced and not otherwise compensated.
The iron species may be fed in the form of iron metal and/or an iron compound, in particular iron oxide, iron fluoride, iron oxyfluoride and/or an iron-aluminium alloy.
Advantageously, the cell may comprise an aluminium-wettable cathode which can be a drained cathode on which aluminium is produced and from which it continuously drains, as described in U.S. Pat. No. 5,651,874 (de Nora/Sekhar) and U.S. Pat. No. 5,683,559 (de Nora).
Usually, the cell is in a monopolar, multi-monopolar or bipolar configuration. The bipolar cell comprises a terminal cathode facing a terminal anode and thereinbetween at least one bipolar electrode, the anode(s) described above forming the anodic side of the or each bipolar electrode and/or of the terminal anode.
In such a bipolar cell an electric current is passed from the surface of the terminal cathode to the surface of the terminal anode as ionic current in the electrolyte and as electronic current through the bipolar electrodes, thereby electrolysing the alumina dissolved in the electrolyte to produce aluminium on each cathode surface and oxygen on each anode surface.
Preferably, the cell comprises means to improve the circulation of the electrolyte between the anodes and facing cathodes and/or means to facilitate dissolution of alumina in the electrolyte. Such means can for instance be provided by the geometry of the cell as described in copending application PCT/IB99/00222 (de Nora/Duruz) or by periodically moving-the anodes as described in co-pending application PCT/IB99/00223 (Duruz/Bellò).
Yet another aspect of the invention is a method of producing aluminium in a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte having at least one anode as described above facing at least one cathode. The method comprises dissolving alumina in the electrolyte and passing an ionic electric current between the electrochemically active surface of the anode(s) and the surface of the cathode(s), thereby electrolysing the dissolved alumina to produce aluminium on the cathode surface(s) and oxygen on the anode surface(s).
Yet a further aspect of the invention is a method of manufacturing an anode and producing aluminium in an electrolytic cell comprising inserting an anode precursor as described above into the electrolyte of an electrolytic cell and forming the iron oxide-based layer to produce a fully manufactured anode and electrolysing alumina in the same (or nearly the same) electrolyte or in a different electrolyte to produce oxygen on the surface of the electrochemically active iron oxide-based layer and aluminium on a facing cathode.
The thus-produced anode may then be transferred from the electrolytic cell in which it was produced to an aluminium electrowinning cell. Alternatively the composition of the electrolyte in which the anode was produced can be suitably modified, for instance by dissolving alumina and optionally iron species, and electrolysis continued in the same cell to produce aluminium.